The over-all objective is to deduce the genetic mechanisms which control the fundamental body segmentation plan of a higher organism; namely, a division of the body into head, thoracic and abdominal segments. The bithorax gene complex in Drosophila will be used for this purpose since extensive studies already have shown that it controls the major pathways by which certain thoracic, abdominal and possibly head patterns of differentiation are brought about. The proposal is (1) to identify and characterize additional gene functions within the bithorax gene complex by means of analyzing the effect of partial deletions (homozygous) of the complex not only on the cuticle but also on the internal organ systems, as well; (2) to analyze two regulatory-like mutants which are located within the gene complex and which apparently activate certain specific genes whose existence has been discovered in the course of the aforementioned partial gene deletion analysis; and (3) to synthesize an unstable ring X-chromosome containing a duplication for the birthorax gene complex (Dp-P68) which will enable somatic mosaic studies to be carried out with not only mutants but also homozygous deletions for the entire bithorax gene complex, or portions thereof.